The third generation partnership project (3GPP) is currently working on standardization the next generation of mobile communication system denoted Long Term Evolution (LTE). The architecture of the LTE system is shown in FIG. 1. In FIG. 1 the logical interfaces (S1) between the evolved Node Bs (eNBs) and the Mobility Management Entities (MME)/Serving Gateway (S-GW) and the interfaces (X2) between the eNBs are shown.
In LTE the downlink is based on orthogonal frequency division multiplexing (OFDM) while the uplink is based on a single carrier modulation method known as discrete Fourier transform spread OFDM (DFT-S-OFDM), see 3GPP TR 36.300, Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); Overall description; Stage 2, V8.2.0.
In LTE distributed self-optimization mechanisms are provided. The mechanisms partly aim at adapting parameters to the cell size by observing cell performance via User Equipment (UE) and eNB measurements. Some existing mechanisms are briefly described below.
Load Balancing
Load balancing can be separated into short term (local) and long term (global), where the former aims at compensating for short term traffic changes, and the latter for more permanent load differences between cells. Short term load balancing can be aware of the long term load balancing and vice versa.
Typically, long term load balancing is closely related to or equivalent to centralized cell size changes. This can be accomplished by adjusting the antenna orientation, or the pilot signal (known as reference signal in LTE) power upon which cell selection is based. It is also possible to specify cell specific offsets considered in the cell selection procedure as a soft means to adjust the cell size. A centralized long term cell size update can preferably be directly incorporated into the short term load balancing, which aims at adjusting short term cell sizes to balance the traffic between cells, for example by adjusting cell selection offsets, or by moving specific user links between cells.
If the cell size of a neighbour cell to a considered cell is changed, then distributed load balancing in the considered cell is affected. Below some exemplifying size-dependent parameters are discussed in the following subsections. The cell size can be determined by the coverage of a service corresponding to a minimum quality of service level. With appropriately configured cells, this is about the same coverage as the reference signal (pilot signal) and the broadcast information (e.g. system information). Furthermore, the cell size can also be determined by signal round-trip time limitations.
Automatic Neighbour Cell Relations
Each cell in the network is identified by a globally unique identity GID, and a locally unique physical cell identity PCID. The former is a unique bit string signalled in the system information, while the latter is an integer (0-503 in LTE) associated to a physical reference signal sequence which the mobile can use to identify a cell on the physical layer. When a mobile station discovers a candidate cell it reports PCID of the cell to its serving cell. If this PCID is unknown to the serving cell it can request the mobile station to decode and report the globally unique GID of the cell to uniquely identify it. This enables neighbour cell relation lists to be established automatically.
The PCIDs are not globally unique, but with careful assignments, they can be locally unique which means that the mobile can report a candidate cell by its PCID, and the serving cell can determine the likely cell if the PCID is listed in the serving cells neighbour cell relation list, and initiate handover to this cell.
When the long term cell size is adjusted, the set of appropriate neighbour cell relations may be different. Some new neighbour cells may be discovered, and some existing neighbour cells may not be needed anymore.
PCID Conflict Detection and Resolution
If a serving cell has two cells A and B in the vicinity with the same PCID, and only A is listed in the cell relation list, then a mobile reporting cell B will be handed over to cell A.
The consequence is most likely a handover failure. If the mobile is also requested to report the global cell identity of cell B, then the serving cell can detect that there is a PCID conflict between these cells, and that PCIDs cannot uniquely identify those cells. Such conflicts need to be resolved. The existence of PCID conflicts are more probable directly after the network has been reconfigured, for example if one or several cells have changed the cell size.
Downlink and Uplink Control Channel Configurations
The coverage of the downlink and uplink control channels is determined by power levels, signal durations and other configurations with the objective to match the service area of the cell. When the long term cell size is reduced, it may be possible to reduce the signalling power or resources while maintaining service area coverage. Similarly, the signalling power and resources may needs to be increased if the cell service area is increased. This is further exemplified by the random access procedure in the next subsection.
Random Access Procedure in LTE
During initial access, the UE seeks access to the network in order to register and commence services. The random access (RA) serves as an uplink control procedure to enable the UE to access the network.
FIG. 2a shows the detailed timing of the basic random-access preamble. The preamble is prefixed with a cyclic prefix (CP) to enable simple frequency domain processing. Its length is in the order of TGP+TDS, where TDS corresponds to the maximum delay spread and TGP corresponds to the maximum round trip time. The CP insures that the received signal is always circular (after removing the CP in the RA receiver) and thus can be processed by Fast Fourier Transforms FFTs.
FIGS. 2b to 2d show the extended preamble formats. Format 1 has an extended CP and is suited for cell radii up to approximately 100 km. However, since no repetition occurs this format is only suited for environments with good propagation conditions. Format 2 contains a repeated main preamble and a cyclic prefix of approximately 200 μs. This format supports cell radii of up to approximately 30 km. Format 3 also contains a repeated main preamble and an extended CP. Using a RA opportunity length of 3 ms this format supports cell radii of up to approximately 100 km. In opposite to format 1 format 3 contains a repeated preamble and is therefore better suited for environments with bad propagation conditions.
The Time Division Multiple Access/Frequency Division Multiple Access TDMA/FDMA structure of the Radio access RA opportunities in Frequency Division Duplex FDD is visualized in FIG. 3. Here only one 1.08 MHz band is allocated to RA at each time whereas several bands are possible in case of Time Division Duplex TDD. The RA opportunities always occur at the band edges of the physical uplink shared channel directly adjacent to the physical uplink control channel.
Power control has been agreed for RACH in LTE, see 3GPP TR 36.300, Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access (E-UTRAN); Overall description; Stage 2, V8.2.0:PRACH(N)=min{PMAX,PO—RACH+PL+(N−1)ΔRACH+ΔPreamble}.
where                PRACH is the preamble transmit power,        N=1, 2, 3, . . . is the RACH attempt number        PMAX is the maximum UE power,        PO—RACH is a 4-bit cell specific parameter signalled via BCH with a granularity of 2 dB (difference in maximum and minimum PO—RACH is 30 dB)        PL is the path loss estimated by the UE        ΔRACH is the power ramping step signaled via BCCH and represented by 2 bits (4 levels) with a granularity of 2 dB        ΔPreamble is a preamble-based offset (format 0-3)        
The UE will increase its transmission power until network access is granted. There is typically an upper bound on the number of retransmissions and, thus, number of power increases. The behaviour of the power control depends on the cell size, since the cell-wide uncertainty in the downlink path loss measurements and associated applicability for the uplink increases with cell size.
An important trade-off in any kind of control system is between responsiveness to sudden and abrupt changes, and insensitivity to noise. The latter can be handled by filtering and long data aggregation before considering measurement information inputs. This naturally reduces responsiveness.
Hence, there exist a need for a method and a device that enables an improved control system that is both responsive and insensitive to noise.